1. Field of the Invention
The field of the invention relates to methods of calculating approach trajectories for aircraft. The object of the invention is to optimize an aircraft's approach trajectory to an airport so as to best limit the environmental nuisance above the airport zones. This nuisance is essentially noise and greenhouse gas emissions. Terminal procedures for takeoff or landing have been the subject since the 1990s of research to decrease this nuisance.
2. Description of the Prior Art
FIG. 1 represents various approach profiles for landing an aircraft A on a landing runway ARR. The non-optimized current procedures comprise alternate plateaus and descent segments as indicated in the white staircase curve of FIG. 1. This vertical trajectory is called “Current”. The plateaus P allow the air traffic controller to monitor and separate the aeroplanes in relation to the relief and in relation to the other aeroplanes by altitude criteria. They allow the aeroplane to decrease its speed, and to change its aerodynamic configuration by extending the slats and flaps while guaranteeing passenger comfort.
The introduction of new less noisy plateau-free approach procedures leads to constructions of very optimized vertical profiles where the margin of manoeuvre for recapturing the vertical plane in the event of deviation is very small. Two optimized approach profiles are represented in FIG. 1. The two profiles are representative of plateau-free vertical procedures, better known by the terminology CDA, the acronym standing for Continuous Descent Approach.
The vertical slopes of these procedures are pegged since the speed and the thrust of the aircraft are pegged. In these procedures, the thrust is in general pegged at a low value, close to the engine “idle” speed, also called the “Idle Thrust”, so as to decrease noise and audible nuisance. The speeds are likewise fixed, either at their optimal value calculated by the onboard flight management system, also called the FMS, or because of constraints that the controller may have fixed at certain points of the approach. Each aeroplane, having regard to its performance, therefore follows a slope resulting from the economic speed/reduced thrust pair. This descent is termed OPT CONF/FPA. It is represented by a curved strip in FIG. 1.
Moreover, in certain cases, for reasons of determinism in the positioning of the aeroplane, constraints on the vertical trajectory may be fixed by the controller. These waypoint constraints C are represented in FIG. 1 by two inverted triangles with common apex. The FMS then limits the optimization of the profile to the determination of the speeds corresponding to a minimum thrust. This descent is termed OPT CONF. It is represented by a straight strip in FIG. 1. For most craft, the resulting thrust is then greater than the reduced thrust so as to enable the prescribed trajectory to be followed.
The onboard flight management system termed FMS is the computer which determines the geometry of the vertical profile, and dispatches the guidance setpoints for following this profile to the pilot or to the automatic pilot. FIG. 2 represents an FMS having available the functions described in the AEEC standard, the acronym standing for Airlines Electronic Engineering Committee, bearing the reference ARINC 702A and entitled “Advanced Flight Management Computer System”. This system comprises notably:                the navigation functions dubbed “LOCNAV” 170, for performing optimal location of the aircraft as a function of the geo-locating means 60 which can be, for example, geo-satellite locating means of GPS or GALILEO type, VHF radio beacons, inertial platforms, etc.;        the flight plan determination functions dubbed “FPLN” 110, making it possible to input the geographical elements constituting the skeleton of the route to be followed and which are the departure and arrival procedures, the waypoints also called “airways”;        the navigation database dubbed “NAVDB” 130, for constructing geographical routes and procedures using data included in the bases, these data being points, beacons, interception or altitude segments called “legs”, etc.;        the performance database, dubbed “PRF DB 150”, containing the aerodynamic parameters and the performance of the engines of the craft;        the functions for determining lateral trajectory dubbed “TRAJ”, 120 making it possible to construct a continuous trajectory on the basis of the points of the flight plan, complying with the aeroplane performance and the confinement constraints called “RNP”;        the prediction functions dubbed “PRED” 140, making it possible to construct a vertical profile optimized on the lateral trajectory;        the guidance functions dubbed “GUID” 200, for guiding the aircraft on its 3D trajectory in the lateral and vertical planes, while optimizing the speed. These functions are linked to the automatic pilot 80;        the functions of digital data links “DATALINK” 180 for communicating with the control centres 70 and the other aircraft.        
The FMS as a whole is linked to man-machine interfaces 50 such as screens, keyboards, designators, etc.
The FMS operates as follows. The flight plan is entered by the pilot or through a data link using data contained in the navigation database. It consists of a succession of segments called “legs” which are formed of a termination and of a geometry such as turn, great circle, rhumb line, etc. These legs are standardized at the international level in an AEEC document bearing the reference ARINC 424.
The pilot thereafter enters the aeroplane parameters such as its mass, the cruising levels, the optimization criteria allowing the modules TRAJ and PRED to calculate respectively the lateral trajectory and the vertical profile in terms of altitude and of speed of the aircraft. The vertical profile is built on the lateral trajectory and therefore changes with the latter.
The major problem which today prevents massive deployment of continuous descent approach procedures, stems from the way in which controllers must operate and the current process for constructing the lateral segments for approach procedures. Specifically, during these phases, it very frequently happens that the structure of the flight plan of the approach contains only legs fixing the trajectory of the aeroplane with respect to the ground in a deterministic manner. Thus there exist legs of semi-infinite “half line” type, which start from a point, along a course or a route and go off to infinity. They are used by the control to “place” the aeroplanes on a “rail” and thereafter string them out to make them follow the final approach segment with optimized spacings for runway occupancy. This is illustrated in FIG. 3 which represents a view in a horizontal plane of an approach flight plan comprising various legs, including a semi-infinite leg. In this figure and in the following, the ends of the legs are represented by crosses denoted WPTi and the landing runway by a white rectangle denoted ARR. In general, the FMS then constructs the vertical profile by making the assumption that the “semi infinite” leg terminates when the orthogonal projection of the following leg is reached. In the case of FIG. 3, the instruction is firstly to follow the flight plan (WPT1 . . . WPT5), then to continue on the course starting from WP5 dashed in FIG. 3. The controller contacts the aeroplane at the right moment to order it to turn around so as to attain the final approach at WPT6, through a DIRECT TO instruction, for example.
It also very frequently happens in the approach phase that the aeroplane is tracked and controlled by radar. One then speaks of “radar vectoring”. Specifically, having regard to the high traffic density in the environs of aerodromes, and the necessity to string out the aeroplanes at a sustained rhythm on the final segment with a view to landing, the controller takes the aeroplanes off their optimized 3D trajectory so as to slow them down or accelerate them as illustrated in FIG. 4. The controller has three degrees of freedom. He can dispatch course, speed and altitude setpoints to the aeroplane. The course setpoints are the ones most used in the approach, in order to shorten or conversely to lengthen the flight plan. The speed and altitude setpoints are degrees of freedom that are more complex since, ultimately, to land at the right speed and at the required runway-altitude, ensuring a fixed temporal separation between the aeroplanes, it is necessary to bring the aeroplane to the entrance to the final segment with the right speed and the right altitude. In the case of “radar vectoring” represented in FIG. 4, the aeroplane is put on course by the air traffic controller. It leaves the flight plan. The course is represented dashed ahead of the aeroplane. At the right moment, the controller orders a DIRECT TO, possibly with an interception course on the point WPT6.
The “nominal” vertical profile is calculated starting from the lateral flight plan. If the latter includes lateral discontinuities, they are processed “conventionally”, the predictions are made by assuming a great circle between the points upstream and downstream of the discontinuity. For the semi-infinite legs, we project orthogonally onto the following leg to determine a nominal lateral flight plan as illustrated in FIGS. 5 and 6 which represent a view from above and a lateral view of an approach flight plan. In these figures, the point Proj corresponds to the projection of the semi-infinite leg starting from the point WPT5 onto the point WPT6, the starting point of the following leg.
In a large majority of cases, the lateral trajectory therefore evolves in a quasi systematic manner for all the aeroplanes in the approach. Consequently, the vertical profile optimized to reduce nuisance cannot be held.